A spectrometer is a device which receives a light signal as an input and produces as an output a light signal which is spread out, or dispersed, in space according the different wavelength components, or colors, of the input light signal. A detector attached to the spectrometer can analyze the output signal, called the spectrum, in order to quantify the amount of each wavelength component present in the input signal.
Spectrometers are used in myriad scientific and industrial applications. For example, they are used for the precise determination of color; such an application known as colorimetry. There are many commercial applications in which the precise knowledge of the color content of a sample material is vital to the successful outcome of a project. For example, in the automotive industry, exact color matching is essential when a portion of a vehicle is being painted so that the repainted portion matches the original color of the rest of the vehicle; the ability to repaint only the repaired portion of the vehicle rather than repainting the entire vehicle leads to considerable savings of money, materials, and time. Another example is in the aerospace industry; if an engineering determination is made for an optimum color scheme for an aircraft which minimizes its ability to be detected by the enemy, it is vital that the exact color specified can be provided by the paint supplier. These are only two of many possible examples of the importance of proper color matching in industrial applications.
Regardless of their specific application, it is important for spectrometers to be capable of preventing noise from interfering with the desired sample measurement. Two noise sources which represent a significant problem in state-of-the-art spectrometers are stray light and re-entrant spectra: stray light refers to any light arriving at the spectrometer output which results from anything other than the spectral dispersion of the input signal; re-entrant spectra refers to spectra resulting from multiple reflections off of the grating between the detector and entrance slit, thus giving rise to unwanted secondary spectra. It is appreciated that stray light and re-entrant spectra represent obstacles to achieving an improved signal-to-noise ratio, thus limiting measurement accuracy or sensitivity, dynamic range, and/or acquisition speed, while also generally requiring that applications using any excitation optical signal (e.g., colorimetry) must generally operate at higher power for a given acquisition or sampling time.
There is a need, therefore, for further improvements in spectrometers, and particularly, for a spectrometer which eliminates or substantially reduces noise, such as stray light and re-entrant spectra, in order to provide an improved signal-to-noise ratio.